The incoming head of the Crown Prosecution Service was once convicted of possessing cannabis, the Attorney General's Office has confirmed.

Ken Macdonald QC was an 18-year-old student when he sent 0.1g of the drug - worth 25p - through the post to a friend in December 1971.

Mr Macdonald, who takes over as Director of Public Prosecutions in November, was fined #75 with #5 costs.

His conviction, and a speeding conviction in 2001, were known before his appointment, the Attorney General's Office said in a statement.

However, Tony Blair has already faced allegations of "rampant cronyism" for putting a fellow founder member of his wife Cherie Booth QC's legal chambers Matrix in the top post.

Shadow chancellor Michael Howard savaged Mr Macdonald's record following his appointment last week, saying his career, rather than his link with the Prime Minister's wife, made him unsuitable for the post.

Shadow home secretary Oliver Letwin repeated those misgivings.

"There may be public disquiet about the appointment of a chief prosecutor who has himself been prosecuted in the past," he said.

"We are more concerned about the fact that, as Michael Howard pointed out several days ago, the Government has nominated somebody who has very little by way of experience of prosecuting ordinary criminals and who has specialised in defending those accused of what he terms `political violence'."

Mr Macdonald takes over the #145,000-a-year post from Sir David Calvert-Smith.

In the state, the Attorney General's Office confirmed he had "been convicted of possessing and procuring 0.1g of cannabis over 30 years ago when he was an 18-year-old student, and of driving at 60mph in a 50mph zone in 2001".

"These convictions were disclosed before the appointment was made.

"The cannabis conviction dates from 1971, and has been a spent conviction since 1976.

"It was also declared when Mr Macdonald applied to join the Bar and when he applied to become a QC, and when he was appointed a part-time judge in 2001.

"It was not a bar to any of these appointments."

Mr Macdonald pleaded guilty to the drugs charge at Oxford Magistrates Court in December 1971.

In February 2001 he was fined #60 and received three points on his driving licence for driving at 60mph in a 50mph zone.

Mr Macdonald was educated at grammar school and at St Edmund Hall, Oxford. He was called to the Bar in 1978 and took silk in 1997. As well as being a founder member of Matrix Chambers he was chairman of the Criminal Bar Association and has sat as a part-time crown court judge since 2001.